Process for the Preparation of Lower Olefins from Heavy Wax

ABSTRACT

The present invention relates to a process for the preparation of lower olefins from a heavy synthetic oil fraction prepared in a Fischer Topsch process. The process according to the invention comprises mild thermal cracking of the heavy synthetic oil fraction, followed by short residence, high temperature thermal cracking of the product obtained in the first process.

The present invention relates to a process for the preparation of lowerolefins from a heavy synthetic oil fraction prepared in a FischerTropsch process. The process according to the invention comprises mildthermal cracking of the heavy synthetic oil fraction, followed by shortresidence, high temperature thermal cracking of the product obtained inthe first process.

Lower olefins, i.e. olefins having from 2 to 4 carbon atoms, moreparticularly ethene and/or propene, are very suitable as startingmaterials in a large number of chemical processes, such as alkylation,oligomerization and polymerisation processes. At the present moment, themain commercial preparation process for these lower olefins is a thermalcracking process in which a hydrocarbon feed, especially ethane or crudeoil derived naphtha, is thermally cracked in a short residence, hightemperature thermal cracking step. The thermal cracking process, alsocalled pyrolysis process, is a gas phase process and is usually carriedout in the presence of an inert gas, often steam or nitrogen, especiallysteam. In the latter case reference is often made to a steam crackingprocess. Such a process is commercially applied in a large number ofpetrochemical complexes.

There is a clear interest in hydrocarbon feedstreams which show a highselectivity towards lower olefins and avoiding as much as possible theformation of methane and/or higher hydrocarbons, especially aromatichydrocarbons. In addition, the formation of coke should be avoided asmuch as possible, while a high conversion of the hydrocarbon feedstreamshould be obtained.

In EP 161 705 it has been disclosed that a fraction of the product of aFischer Tropsch process may be used as a hydrocarbon feed in a thermalcracking process. This reference especially relates to the preparationof C₁₀₋₂₀ olefins from the C₂₀₊ hydrocarbon fraction made in the FischerTropsch process by mild thermal cracking. The C¹⁹⁻ fraction may beconverted into lower olefins by means of steam cracking. The lowerolefins thus prepared can be oligomerized to prepare a mixture ofolefins which consists partly of C₁₀-1C₂₀ olefins.

In EP 584 879 it has been described that the selectivity of a steamcracking process towards lower olefins, also at high conversion levels,can be further and significantly increased when use is made of asynthetic oil fraction such as a Fischer Tropsch product as ahydrocarbon feed in the steam cracking process, which synthetic oilfraction has been hydro-processed. Suitable hydrogenation processesinclude hydrogenation, hydroisomerisation and/or hydrocracking.Preferred boiling ranges of the starting material are between 30 and350° C., more preferably between 30 and 200° C.

In the known processes in which a Fischer Tropsch fraction is used as afeed for a thermal cracking process to prepare lower olefins, the feedis essentially a non-residual feed, i.e. the feed does not contain anyhydrocarbons boiling above 500° C., or even 600° C. In the case thatcertain amounts of very heavy hydrocarbons would be present in the feed,e.g. more than a few wt percent of material boiling above 500 or even600° C., such a feed is not very suitable any more for a steam crackingprocess, as such feed streams would result in large amounts of liquidproduct during the short residence time high temperature thermalcracking process. This will result in a large amount of unconvertedfeed. In addition, more coke formation may occur, thus requiring morefrequent decoking operations.

It has now been found that in the case in which larger amounts of veryheavy hydrocarbons are present in a Fischer Tropsch feed, especiallymore than 5 wt % based on total feed of material boiling above 550° C.,such a feed, before being processed in a short residence time, hightemperature thermal cracking process, is advantageously pre-treated in amild thermal cracking process. During such a pre-treatment especiallythe largest hydrocarbon molecules are cracked, resulting in aconsiderable decrease of the amount of 550° C.+material in the feed. Afeedstream thus being treated can be used without further problems in ashort residence time, high temperature thermal cracking process, e.g. asteam cracking process, for conversion into lower olefins, i.e. olefinshaving from 2 to 4 carbon atoms, more particularly ethene and/orpropene. In this way a very high yield of lower olefins is obtained, theamount of undesired by-products as methane and/or aromatic compounds, isrelatively low and coke formation is also relatively low.

Thus, the present process concerns a process for the preparation oflower olefins from a synthetic oil fraction prepared in a FischerTropsch process, the process comprising mild thermal cracking of thesynthetic oil fraction, followed by short residence time, hightemperature thermal cracking of the product obtained in the mild thermalcracking step, in which process at least 5 wt % of the synthetic oilfraction has a boiling point above 550° C.

The process of the invention makes it possible to use the completehydrocarbonaceous product, including the fraction boiling above 550° C.,or even boiling above 650° C., of a Fischer Tropsch process for theproduction of lower olefins. In the usual commercial processes formaking lower olefins from hydrocarbon feedstocks, it is difficult to usematerial boiling above 550° C. and certainly material boiling above 650°C. In the present process this high boiling material is converted intolower boiling material, thus making it better suitable as steamcrackerfeed. Lower olefins can be made in a very high yield due to a highconversion of the feed in combination with a high selectivity for lowerolefins. In addition, coke formation is low in comparison with crude oilbased naphtha fractions. Methane formation as well as the formation ofhigher hydrocarbons, especially aromatic hydrocarbons is relative low.

The overall conversion of the starting synthetic oil fraction into lowerolefins, i.e. C₂-C₄ olefins, varies between 40 and 80% based on theweight of the starting material and the weight of the lower olefinsproduced, usually between 55 and 70%.

The process of the present invention is especially suitable forfeedstocks in which the part of the synthetic oil fraction having aboiling point above 550° C. is at least 10 wt % of the total syntheticoil fraction, preferably 30 wt %, more preferably at least 50 wt %.

In one embodiment of the invention the complete Fischer Tropsch productis sent to the mild thermal cracking step. In a preferred embodimentonly the C₅₊ fraction is sent to the mild thermal cracking step. Thisseparation can be done easily by cooling down the product of the FischerTropsch reaction and separating the gaseous fraction from the liquidfraction. By choosing the temperature and the pressure of the reactionproduct correctly, a separation can be made between unconverted syngas,inerts and C₁ to C₄ hydrocarbons and the C₅₊ fraction. Such a separationhas the additional advantage that also Fischer Tropsch product waterwill be removed from the feed product. By choosing the temperatureand/or the pressure differently, it is also possible to isolate the e.g.C₃₊ fraction or the C₁₀₊ or the C₂₀₊ fraction. In a further preferredprocess only the more heavy products from the Fischer Tropsch processare sent to the mild thermal cracking process, e.g. only the 250° C.fraction, or more preferably the 350° C. fraction.

The present process is especially suitable for Fischer Tropsch reactionproducts which are made under such circumstances that an extremely heavyproduct is obtained. In order to minimise the amount of C₁-C₄ productwhich is formed in the Fischer Tropsch process, conditions and catalystare used nowadays in which substantial parts of the Fischer Tropschproducts exists of material boiling above 650° C. Thus, in a preferredembodiment the starting synthetic oil fraction has a boiling point above650° C., the amount boiling above 650° C. suitably being at least 10 wt% of the total synthetic oil fraction, preferably at least 20 wt %, morepreferably at least 40 wt %, still more preferably at least 60 wt %. Ina further preferred embodiment the starting synthetic oil fractioncomprises at least 10 wt % of material boiling above 750° C. based onthe total synthetic oil fraction, preferably at least 40 wt %, morepreferably at least 70 wt %.

The product of the Fischer Tropsch reaction mainly consists of paraffin,olefins and oxygenates. These compounds in general have the same carbonchain structure, the difference being the functionality of the atomsattached to the carbon chain. Under exceptional circumstances naphthenicand/or aromatic compounds may be formed (especially when relatively hightemperatures are used), however, these compounds are not desired. It ispreferred that the Fischer Tropsch compounds are straight chaincompounds, or straight chain compounds having up to three, preferably upto two, more preferably up till one branch, preferably methyl or ethylbranches, especially methyl branches. Straight chain, i.e. normalparaffins, are preferred. Thus, preferably the synthetic oil fraction tobe used in the claimed process, after hydrogenation, comprises at least90 wt % of paraffins, preferably 95 wt %. The amount of naphthenic andaromatic compounds together is preferably at most 5 wt %, morepreferably at most 1 wt %. The amount of normal compounds (paraffins,olefins and oxygenates) is suitably as high as possible. Thus, thesynthetic oil fraction comprises at least 50 wt % normal compounds,preferably at least 65 wt %, more preferably at least 80 wt %. Theamount of olefins in the synthetic oil fraction is suitably less than 30wt % of total fraction, preferably less than 20 wt %, more preferablyless than 10 wt %. The amount of oxygenates is suitably less than 15 wt% based on total fraction, preferably less than 7.5 wt %, morepreferably less than 5 wt %.

In a further preferred embodiment the synthetic oil fraction ishydroprocessed before the mild thermal cracking process, preferablyhydrogenated. This results in a further increase of the yield ofethene/propene. Hydrogenation is suitable being carried out at elevatedtemperature and pressure in the presence of hydrogen and a hydrogenationcatalyst. The action of the hydrogenation stage is, for example, tohydrogenate any unsaturated hydrocarbons and oxygenates present in thesynthetic oil without substantial hydroisomerisation and/orhydrocracking occurring. Preferably, the hydrogenation is carried out ata temperature of from 100° C. to 300° C., more preferably at atemperature of from 150° C. to 275° C., in particular of from 175° C. to250° C. The hydrogenation may be carried out at a relatively wide rangeof pressures, but preferably, the hydrogenation is carried out at ahydrogen partial pressure of from 5 bar to 150 bar, more preferably offrom 20 bar to 120 bar.

The hydrogenation may be carried out using any type of catalyst bedarrangement, such as a fluidized bed, moving bed, slurry phase bed or afixed bed, each type of catalyst bed having its own characteristicadvantages and disadvantages. However, preferably a fixed catalyst bedis applied. It is to be understood that the reaction conditions, such astemperature, pressure and space velocity, may vary according to thespecific type of catalyst bed being used. If a fixed catalyst bed isbeing used, the synthetic oil feed is preferably provided at a weighthourly space velocity of from 0.1 kg/l/h to 5 kg/l/h, more preferably ata weight hourly space velocity of from 0.25 kg/l/h to 2.5 kg/l/h.Hydrogen may be applied to the hydrogenation stage at a gas hourly spacevelocity in the range of from 100 to 10000 Nl/l/hr, more preferably from250 to 5000 Nl/l/hr. The ratio of hydrogen to the feed may range from100 to 5000 Nl/kg and is preferably from 250 to 2500 Nl/kg.

Hydrogenation catalysts are well known in the art and are commerciallyavailable in a large variety of compositions. Typically, thehydrogenation catalyst comprises as catalytically active component oneor more metals selected from Groups VIb and VIII of the Periodic Tableof the Elements, in particular one or more metals selected frommolybdenum, tungsten, cobalt, nickel, ruthenium, iridium, osmium,platinum and palladium. Preferably, the catalyst comprises one or moremetals selected from nickel, platinum and palladium as the catalyticallyactive component. A particularly suitable catalyst comprises nickel as acatalytically active component.

Hydrogenation catalysts typically comprise a refractory metal oxide orsilicate as a carrier. Suitable carrier materials include silica,alumina, silica-alumina, zirconia, titania and mixtures thereof.Preferred carrier materials for inclusion in the hydrogenation catalystare silica, alumina and silica-alumina.

The hydrogenation catalyst may comprise the catalytically activecomponent in an amount of from 0.05 to 70 parts by weight, preferablyfrom 0.1 to 50 parts by weight, calculated as metal(s) per 100 parts byweight of total catalyst. The amount of catalytically active metalpresent in the catalyst will vary according to the specific metalconcerned. A particularly suitable hydrogenation catalyst comprisesnickel in an amount in the range of from 30 to 70 parts by weight,calculated as metal per 100 parts by weight of total catalyst.

Suitable hydrogenation catalysts are available commercially, or may beprepared by methods well known in the art, for example comulling,impregnation or precipitation.

In the case that more severe conditions are used for the hydrogenationreaction, e.g. temperatures between 275 and 400° C., especially incombination with acidic carriers as amorphous silica/alumina andespecially zeolites, also hydroisomerisation will occur. In general theamount of hydroisomerisation will be kept less than 40 wt % based onfeed, preferably less than 10 wt %, more preferably less than 5 wt %. Inall cases, hydrocracking is to be kept at a relatively low level. Thus,the conversion of any material boiling above 360° C. into materialboiling below 360° C. is suitably less than 15 wt %, preferably lessthan 10 wt %, more preferably less than 3 wt %.

In another embodiment oxygenates may be removed by a dehydrationprocess. This process is suitably carried out in the presence of adehydration catalyst. Suitable dehydration catalysts are well known inthe art. Especially acid catalysts are used. Removal of oxygenatesresults in a higher yield of ethene/propene, while the oxygen content ofthe ethene/propene fraction will be deceased. This is important as incertain catalytic polymerization processes the catalyst may very verysensitive to even trace amounts of oxygen.

The initial boiling point of the synthetic oil to be used in the processaccording to the invention is suitably at least 30° C. In a preferredembodiment the initial boiling point is at least 250° C., moreespecially at least 350° C., more preferably at least 450° C., even morepreferably at least 55020 C. By removing a part of the low boilingfraction, a smaller thermal cracking unit may be used. The untreatedinitial feed, especially the feed boiling between 30 and 350, preferably450, more preferably 550° C., may be combined with the thermally crackedfraction, and the combined stream can be used as feed for the shortresidence time/high temperature thermal cracking process.

The mild thermal cracking process to be used in the process according tothe present invention may be any mild thermal cracking process known inthe art. Very suitably it is done by a furnace cracking process, but itis preferably a soaker visbreaking process. In the soaker visbreakingprocess the feed is heated in a furnace to a temperature suitablybetween 380 and 500° C., preferably between 400 and 480° C., suitablyusing a residence time of up till 5 minutes, preferably up till 3minutes, followed by further conversion in a soaker vessel. Theresidence time in the soaker vessel is suitably between 0.5 and 2 hours.The pressure is usually between 3 and 10 bar. The conversion (ofmaterial boiling above 550° C.) obtained is suitably at least 20 wt %,preferably at least 60 wt %. Especially the conversion is between 30 and98 wt % of the material boiling above 550° C., preferably between 60 and95 wt %. Preferably at least 99 wt % of the material boiling above 750°C. is removed, more preferably at least 99 wt % of the material boilingabove 650° C. is removed. In the case of furnace cracking thetemperature is suitably between 420 and 540° C., preferably between 460and 520° C., the pressure is suitably between 5 and 50 bar, preferablybetween 15 and 20 bar and the residence time is suitably between 1 and15 minutes, especially between 4 and 12 minutes. The conversion levelsare the same as for the soaker process.

The product obtained in the mild thermal cracking process may be useddirectly in the short residence, high temperature cracking process, butis preferably separated into a light fraction and a heavy fraction. Theseparation can be done by means of any equipment known in the art toseparate feedstreams into fractions having different boiling ranges,e.g. distillation equipment, but is preferably done by means of a flashseparation. The light fraction suitably boils up till 450° C.,preferably up till 500°, more preferably up till 550° C. or even 650° C.The heavy fraction may be recycled to the mild thermal cracking step. Inthe case of a recycle it is preferred to remove between 5 and 40 wt % ofthe stream as a bleed stream. Such a bleed stream is advantageously usedas fuel, either in the mild thermal cracking step or in the secondcracking step.

In a further embodiment of the invention, the product obtained in themild thermal cracking process may be hydrogenated before introductioninto the short residence, high temperature thermal cracking process.This can be done in exactly the same way as described above (includingall preferred ranges) for the hydrogenation of the starting FischerTropsch product. In this way the olefins formed during the mild thermalcracking step are converted into saturated paraffins, resulting in afurther improvement (selectivity to lower olefins, reduced cokeformation) of the second cracking step.

The production of lower olefins, in particular ethene and propene, is ingeneral achieved by pyrolyzing the Fischer-Tropsch derived hydrocarbons.

Pyrolysis comprises steam cracking, which is thermal cracking ofhydrocarbons in the presence of steam and if desired a dilution gas. Theprocess comprises a convection zone, a cracking zone, a cooling zone anda separation zone. The pyrolysis furnace comprises the convection zoneand the cracking zone. The convection zone usually comprises a firstpreheating zone and a second preheating zone. Generally, feed is heatedin the first preheating zone, and dilution gas is added to the feedbefore the (liquid and gas) mixture of feed and dilution gas is sent tothe second preheating zone.

Furnaces designed for treating gasoil and even heavier feed streams willhave a larger heat transfer surface area in the first preheating zonethan furnaces designed for a light feed, e.g. naphtha, as the main aimof the first preheating zone is vaporizing the feed and heating thefeed.

A furnace designed for treating gaseous feed, will have a smaller heattransfer surface area in the first preheating zone than a furnacedesigned for liquid feed as a gaseous feed does not need to bevaporized. It is to be understood that the scope of the steam crackingprocess may include any number and types of process steps between eachdescribed process step or between a described source and destinationwithin a process step.

Usually and preferably, all product of a process step will be subjectedto the next process step. However, it is possible to send only part ofthe product of a process step to the next process step.

Feed can be introduced into the process at further inlets besides thestandard inlet and the inlet where feed is introduced together withsteam and/or dilution gas. However, it is preferred to introduce feedonly at the standard inlet of the convection zone and further feedtogether with steam and/or dilution gas.

Dilution gas (usually steam) can be added at a single inlet, or can beadded via several inlets. However, it is preferred to add dilution gasat a single inlet.

The convection zone generally comprises a first preheating zone and asecond preheating zone between which is located an inlet for steam andoptionally dilution gas. In the first preheating zone, the feed isheated. After the first preheating zone, steam and optionally dilutiongas is added to the feed and the mixture obtained can be heated furtherin the second preheating zone to a temperature just below thetemperature at which cracking starts to occur. The temperature of theproduct obtained from the convection zone will usually be of from 400 to800° C., depending upon the feed, more specifically of from 450 to 750°C.

The pyrolysis furnace may be any type of conventional olefins pyrolysisfurnace designed for pyrolizing heavy feed and operated for productionof lower boiling products such as olefins, especially including atubular steam cracking furnace. The tubes within the convection zone ofthe pyrolysis furnace may be arranged as a bank of tubes in parallel, orthe tubes may be arranged for a single pass of the feedstock through theconvection zone. Within each bank, the tubes may arranged in a coil orserpentine type arrangement. At the inlet, the feed may be split amongseveral tubes, or may be fed to one single pass tube through which allthe feed flows from the inlet to the outlet of the first stagepreheater. Preferably, the first and/or second preheating zone of theconvection zone comprise a multiple pass tubular reactor in which feedis passed through the first and/or the second preheating zone via morethan one tube. Multiple pass tubular reactors often contain tubes havingconnections at their ends leading feed from the one tube to the nexttube until the feed is sufficiently heated to be mixed with dilution gasand be passed to the second preheating zone, or to be sent to thecracking zone.

The pressure and temperature at which the feed is fed to the inlet ofthe first preheating zone is not critical, typically the temperaturewill be of from 0 to 300° C.

The optimal temperature to which the feed is heated in the firstpreheating zone will depend upon the pressure of the feed, and theperformance and operation of the remainder of the process. The productof the first preheating zone will generally have an exit temperature ofat least 150° C. such as 195° C. The upper range on the temperature ofthe feed in the first preheating zone is limited to the point at whichthe stability of the feed is impaired. At a certain temperature, thecoking propensity of the feed increases. This temperature limit wouldapply to both the first and the second preheating zone and all tubes inthese zones. Preferably, the exit temperature of the feed within thefirst preheating zone is not more than 520° C., preferably not more than500° C., more preferably not more than 450° C. or even 400° C.

The heating elements in the first and second preheating zone in theconvection zone is typically a bank of tubes, wherein the contents inthe tubes are heated primarily by convective heat transfer from thecombustion gas exiting from the cracking zone of the pyrolysis furnace,so-called flue gas. However, different heating elements can be used aswell.

The pressure within the first and second preheating zone is notparticularly limited. The pressure is generally within a range of from 4to 21 bar, more preferably of from 5 to 13 bar.

In the process of the present invention part of the heavy hydrocarbonsobtained by Fischer-Tropsch synthesis as the feed is introduced via thestandard feed inlet of the convection zone, and if desired part of thefeed is introduced further downstream in the convection zone.

Steam gas is added to the convection zone. This can be done preferablyin or before the second preheating zone of the convection zone. Otherdilution gas is preferably added at a point external to the pyrolysisfurnace for ease of maintaining and replacing equipment.

The dilution gas is a vapour at the injection point into the convectionzone. Examples of dilution gases are methane, ethane, nitrogen,hydrogen, natural gas, dry gas, refinery off gases, and a vaporizednaphtha. Preferably, the steam is superheated steam.

Typical dilution gas temperatures at the dilution gas/feed junctionrange of from 140° C. to 800° C., more preferably of from 150° C. to780° C., more preferably of from 200 to 750° C.

The pressure of dilution gas is not particularly limited, but ispreferably sufficient to allow injection. Typical dilution gas pressuresadded to the crude oil is generally within the range of from 6 to 15bar.

It is desirable to add steam and optionally dilution gas between thefirst preheating zone and the second preheating zone in an amount whichwill generally be not more than 1 kg of dilution gas per kg of feed.However, there can be circumstances in which a higher amount of dilutiongas can be advantageous.

The mixture of dilution gas and feed is fed to the second preheatingzone where the mixture is heated further. The mixture generallycomprises not more than 50 wt % liquid Fischer-Tropsch hydrocarbons.Preferably not more than 25 wt %, most preferably not more than 10 wt %Tubes of the second preheating zone can be heated by the flue gases fromthe cracking zone of the furnace. In the second preheating zone (superheater), the mix is fully preheated to near or just below a temperatureat which substantial feedstock cracking and associated coke laydown inthe preheater would occur such as 450 to 550° C., preferably 460-500°C., such as 490° C.

Subsequently, the product of the convection zone is sent to the crackingzone. The temperature of the mixture of steam and feed is increasedfurther under controlled residence time, temperature profile and partialpressure. The exit temperature of the product obtained in the crackingzone is generally of from 700 to up to 1000° C. more specifically offrom 750 to 950° C. The pressure is generally within a range of from 2to 25 bar, more preferably of from 3 to 18 bar.

The reactions in the cracking zone are highly endothermic, and thereforea high rate of energy input is needed.

On leaving the cracking zone, the products are generally immediatelycooled. The temperature of the product will usually be reduced to atemperature of from 200 to 700° C., more specifically of from 250 to650° C. to prevent degradation by secondary reactions. Cooling of theproduct obtained in the cracking zone can be done in any way suitable,such as by direct quenching or indirect quenching.

The cooled product is subsequently separated into the desiredend-products. Separation of the desired end-products can start atcooling where heavy components can be removed. Further, during coolingthe gas obtained can be compressed, and acids and water can be removed.Subsequently, the product can be dried and uncracked feed, ethane andpropane may be recovered for recycling as pyrolysis feed. The crackingseverity affects the composition of the product obtained.

Products of an olefins pyrolysis furnace include, but are not limitedto, ethene, propene, butadiene, benzene, hydrogen, and methane, andother associated olefinic, paraffinic, and aromatic products. Ethenegenerally is the predominant product, typically ranging from 15 to 60 %wt, based on the weight of the feed.

In a typical work-up, the product of the cracking zone is cooled withthe help of a water quench, followed by multi-stage compressiontypically in 4 to 6 stages. Before the last compressor stage, the gas istreated with caustic to remove hydrogen sulphide and carbon dioxide.Acetylenes may be hydrogenated with hydrogen-rich compressor gas. Afterthe last compression stage, the cracked gas is typically dehydrated bychilling and dried by use of molecular sieves. Methane and hydrogen canbe removed in a demethanizer. In a demethanizer, the hydrocarbonscontaining 2 carbon atoms are produced overhead and the hydrocarbonscontaining 3 carbon atoms or more is a bottom product. The overheadstream can be hydrogenated to remove acetylene and then fractionated toproduce ethene and ethane. The ethane can be recycled. The bottomproduct can be further fractionated, if appropriate, to remove heavyends including compounds containing 4 carbon atoms or more. The overheadstream from a depropanizer can be hydrogenated to remove methylacetyleneand propadiene, which can be recovered for sale or removed via othermeans. Propene can be obtained as overhead stream from the depropanizer,and the bottom propane fraction can be recycled.

It is a preferred characteristic of the Fischer-Tropsch hydrocarbonsthat they are essentially free of aromatic compounds, nitrogencomprising compounds and sulphur comprising compounds.

The Fischer-Tropsch hydrocarbons to be used according to the inventionas a feed for steam for the production of lower olefins, are produced ina Fischer-Tropsch synthesis. Fischer-Tropsch synthesis of hydrocarbonsis a well known process. In the Fischer-Tropsch synthesis the startingmaterial is a hydrocarbonaceous feed.

The hydrocarbonaceous feed suitably is methane, natural gas, associatedgas or a mixture of C₁₋₄ hydrocarbons. The feed comprises mainly, i.e.more than 90 v/v %, especially more than 94%, C₁₋₄ hydrocarbons,especially comprises at least 60 v/v percent methane, preferably atleast 75 percent, more preferably 90 percent. Very suitably natural gasor associated gas is used. Suitably, any sulphur in the feedstock isremoved.

The partial oxidation of this hydrocarbons feed, producing mixtures ofespecially carbon monoxide and hydrogen, can take place according tovarious established processes. These processes include the ShellGasification Process. A comprehensive survey of this process can befound in the Oil and Gas Journal, Sep. 6, 1971, pp 86-90.

The oxygen containing gas is air (containing about 21 vol. percent ofoxygen), oxygen enriched air, suitably containing up to 70 percent, orsubstantially pure air, containing typically at least 95 vol. % oxygen.Oxygen or oxygen enriched air may be produced via cryogenic techniques,but could also be produced by a membrane based process, e.g. the processas described in WO 93/06041. To adjust the H₂/CO ratio in the syngas,carbon dioxide and/or steam may be introduced into the partial oxidationprocess. Preferably up to 15% volume based on the amount of syngas,preferably up to 8% volume, more preferable up to 4% volume, of eithercarbon dioxide or steam is added to the feed. Water produced in thehydrocarbon synthesis may be used to generate the steam. As a suitablecarbon dioxide source, carbon dioxide from the effluent gasses of theexpanding/combustion step may be used. The H₂/CO ratio of the syngas issuitably between 1.3 and 2.1, preferably between 1.4 and 2.0. Ifdesired, (small) additional amounts of hydrogen may be made by steammethane reforming, preferably in combination with the water shiftreaction. Any carbon monoxide and carbon dioxide produced together withthe hydrogen may be used in the hydrocarbon synthesis reaction orrecycled to increase the carbon efficiency. Additional hydrogenmanufacture may be an option.

The percentage of light hydrocarbonaceous feed which is converted in thefirst step of the process of the invention is suitably 50-99% by weightand preferably 80-98% by weight, more preferably 85-96% by weight.

The gaseous mixture, comprising predominantly hydrogen, carbon monoxideand optionally nitrogen, is contacted with a suitable catalyst in thecatalytic conversion stage, in which the hydrocarbons are formed.Suitably at least 70 v/v% of the syngas is contacted with the catalyst,preferably at least 80%, more preferably at least 90, still morepreferably all the syngas.

The catalysts used in for the catalytic conversion of the mixturecomprising hydrogen and carbon monoxide are known in the art and areusually referred to as Fischer-Tropsch catalysts. Catalysts for use inthe Fischer-Tropsch hydrocarbon synthesis process frequently comprise,as the catalytically active component, a metal from Group VIII of thePeriodic Table of Elements. Particular catalytically active metalsinclude ruthenium, iron, cobalt and nickel. Cobalt is a preferredcatalytically active metal.

The catalytically active metal is preferably supported on a porouscarrier. The porous carrier may be selected from any of the suitablerefractory metal oxides or silicates or combinations thereof known inthe art. Particular examples of preferred porous carriers includesilica, alumina, titania, zirconia, ceria, gallia and mixtures thereof,especially silica and titania.

The amount of catalytically active metal on the carrier is preferably inthe range of from 3 to 300 pbw per 100 pbw of carrier material, morepreferably from 10 to 80 pbw, especially from 20 to 60 pbw.

If desired, the catalyst may also comprise one or more metals or metaloxides as promoters. Suitable metal oxide promoters may be selected fromGroups IIA, IIIB, IVB, VB and VIB of the Periodic Table of Elements, orthe actinides and lanthanides. In particular, oxides of magnesium,calcium, strontium, barium, scandium, yttrium. lanthanum, cerium,titanium, zirconium, hafnium, thorium, uranium, vanadium, chromium andmanganese are most suitable promoters. Particularly preferred metaloxide promoters for the catalyst used to prepare the waxes for use inthe present invention are manganese and zirconium oxide. Suitable metalpromoters may be selected from Groups VIIB or VIII of the PeriodicTable. Rhenium and Group VIII noble metals are particularly suitable,with platinum and palladium being especially preferred. The amount ofpromoter present in the catalyst is suitably in the range of from 0.01to 100 pbw, preferably 0.1 to 40, more preferably 1 to 20 pbw, per 100pbw of carrier.

The catalytically active metal and the promoter, if present, may bedeposited on the carrier material by any suitable treatment, such asimpregnation, kneading and extrusion. After deposition of the metal and,if appropriate, the promoter on the carrier material, the loaded carrieris typically subjected to calcination at a temperature of generally from350 to 750° C., preferably a temperature in the range of from 450 to550° C. The effect of the calcination treatment is to remove crystalwater, to decompose volatile decomposition products and to convertorganic and inorganic compounds to their respective oxides. Aftercalcination, the resulting catalyst may be activated by contacting thecatalyst with hydrogen or a hydrogen-containing gas, typically attemperatures of about 200 to 350° C.

The catalytic conversion process may be performed under conventionalsynthesis conditions known in the art. Typically, the catalyticconversion may be effected at a temperature in the range of from 100 to600° C., preferably from 150 to 300° C., more preferably from 180 to280° C. Typical total pressures for the catalytic conversion process arein the range of from 1 to 200 bar absolute, more preferably from 10 to70 bar absolute. In the catalytic conversion process mainly (at least 70wt %, preferably 85 wt % of C₅₊ hydrocarbons are formed.

Preferably, a Fischer-Tropsch catalyst is used, which yields substantialquantities of normal (and also iso-) paraffins, more preferablysubstantially normal paraffins. A most suitable catalyst for thispurpose is a cobalt-containing Fischer-Tropsch catalyst. TheFischer-Tropsch hydrocarbons comprise generally C₄-C₂₀₀, preferablyC₄-C₁₀₀ hydrocarbons. Normally liquid Fischer-Tropsch hydrocarbons aresuitably C₄₋₂₅ hydrocarbons, especially C₅₋₂₀ hydrocarbons. Thesehydrocarbons are liquid at temperatures between 5 and 30° C. (1 bar),especially at about 20° C. (1 bar), and usually are paraffinic ofnature, while up to 24 wt %, preferably up to 12 wt %, of either olefinsor oxygenated compounds may be present. Depending on the catalyst andthe process conditions used in the Fischer Tropsch reaction, normallygaseous hydrocarbons, normally liquid hydrocarbons and optionallynormally solid hydrocarbons are obtained.

1. A process for the preparation of lower olefins from a synthetic oilfraction prepared in a Fischer Tropsch process, the process comprisingmild thermal cracking of the synthetic oil fraction to form a product,followed by short residence time, high temperature thermal cracking ofthe product obtained in the mild thermal cracking step, in which processat least 5 wt % of the synthetic oil fraction has a boiling point above550° C.
 2. The process according to claim 1, in which the part of thesynthetic oil fraction having a boiling point above 550° C. is at least10 wt % of the total synthetic oil fraction.
 3. The process according toclaim 1, in which at least 20 wt % of the synthetic oil fraction has aboiling point above 650° C.
 4. The process according to claim 1, inwhich the Fischer Tropsch process comprises reaction of carbon monoxideand hydrogen over a cobalt or iron based Fischer Tropsch catalyst, at atemperature between 150 and 300° C. and a pressure between 5 and 100bara.
 5. The process according to claim 1, in which the synthetic oilfraction comprises at least 90 wt % paraffins after hydrogenation. 6.The process according to claim 1, in which at least part of anyoxygenates and/or olefins in the synthetic oil fraction are removed in apre-treatment process.
 7. The process according to claim 1, in which theinitial boiling point of the synthetic oil fraction is at least 250° C.8. The process according to claim 1, in which the mild thermal crackingprocess comprises furnace cracking in which the furnace cracking iscarried out at a temperature between 500° C. and 700° C. and a residencetime up till 6 minutes.
 9. The process according to claim 8, in whichthe conversion of 550° C.+material into 550° C.−material is at least40%.
 10. The process according to claim 1, in which any 550° C.+materialpresent in the product after mild thermal cracking is separated from thereaction product and recycled to the mild thermal cracking.
 11. Theprocess according to claim 1, in which the C₂ and C₃ is also sent to thehigh temperature thermal cracking unit.
 12. The process according toclaim 1, in which methane produced in the Fischer Tropsch reaction andany unconverted hydrogen and carbon monoxide are used as fuel for mildthermal cracking reaction and/or the high temperature thermal cracking.13. The process according to claim 1, in which natural gas condensatesand/or ethane/propane extracted from natural gas sources are also sentto the high temperature cracking process.
 14. The process according toclaim 1, in which at least 30 wt % of the synthetic oil fraction has aboiling point above 550° C.
 15. The process according to claim 1, inwhich at least 40 wt % of the synthetic oil fraction has a boiling pointabove 650° C.
 16. The process according to claim 1, in which thesynthetic oil fraction comprises at least 50 wt % normal compounds. 17.The process according to claim 1, in which the synthetic oil fractioncomprises at least 70 wt % normal compounds.
 18. The process accordingto claim 1, in which the initial boiling point of the synthetic oilfraction is at least 450° C.
 19. The process according to claim 1, inwhich the mild thermal cracking process comprises soaker cracking whichis done at a temperature between 400° C. and 500° C. and a residencetime between 10 and 60 minutes.